1. Field Of The Invention
This invention relates to an apparatus and method for rapid quenching of molten alloy. More particularly, it relates to characteristics of the quenching surface of a casting wheel used in the continuous casting of metallic strip.
2. Description Of The Prior Art
Continuous casting of alloy strip is accomplished by depositing molten alloy onto a rotating casting wheel. Strip forms as the molten alloy stream is attenuated and solidified by the wheel's moving quench surface. For continuous casting, this quenching surface needs to withstand mechanical damage arising from cyclical stressing due to thermal cycling during casting. Means by which improved performance of the quench surface can be achieved include the use of alloys having high thermal conductivity and high mechanical strength. Examples include copper alloys of various kinds, steels and the like. Alternatively, various surfaces can be plated onto the casting wheel quench, surface in order to improve its performance, as disclosed in European Patent No. EP0024506. Details of a suitable casting procedure have been disclosed in U.S. Pat. No. 4,142,571, and the disclosure of that patent is incorporated herein by reference.
Casting wheel quench surfaces of the prior art generally have been of two forms: monolithic or component. In the former, either a solid block of alloy is fashioned into the form of a casting wheel--either with or without cooling channels incorporated therein. The latter consists of two or more pieces which, when assembled, constitute a casting wheel, as disclosed in U.S. Pat. No. 4,537,239. The casting wheel quench surface improvements of the present disclosure are applicable to all kinds of casting wheels.
Casting wheel quench surfaces of the prior art generally have been made from alloy which was cast and mechanically worked in some manner prior to fabricating a wheel/quench surface therefrom. Certain mechanical properties such as hardness, tensile and yield strength, and elongation had been considered, sometimes in combination with thermal conductivity. This was done in an effort to achieve the best combination of mechanical strength and thermal conductivity properties possible for a given alloy. The reason for this is basically twofold: 1) to provide a quench rate which is high enough to result in the cast strip microstructure which is desired, 2) to resist quench surface mechanical damage which would result in degradation of strip geometric definition and thereby render the cast product unserviceable.
An alloy strip casting process is complicated and dynamic or cyclical mechanical properties need to be seriously considered in order to develop a quench surface which has superior performance characteristics. The processes by which the feedstock alloy for use as a quenching surface is made can significantly affect subsequent strip casting performance. This can be due to the amount of mechanical work and subsequent strengthening phases which occur after heat treatment. It can also be due to the directionality or the discrete nature of some mechanical working processes. For example, ring forging and extrusion both impart anisotropy of mechanical properties to a work piece. Unfortunately, the direction of this resulting orientation is not typically aligned along the most useful direction within the quench surface. The heat treatment to achieve alloy recrystallization and grain growth and strengthening phase precipitation with the alloy matrix is often insufficient to ameliorate the deficiencies induced during the mechanical working process steps. The results are a quench surface with microstructure having non-uniform grain size, shape, and distribution.
A consequence of having a quench surface grain structure such as the one described is a predisposition of that component to fail prematurely while in the service of continuously casting alloy strip. As mentioned, the ab initio grain size non-uniformity will result in greatly limited fatigue life of any component for which it is used.